This research proposes to use the Wisconsin Longitudinal Study (WLS) to study several aspects of the interplay between the changing cognitive functioning of study participants and the changing circumstances of their lives. We propose first to provide an extensive assessment of the measurement structure and properties of the WLS cognitive data, and we will use overlapping measures between the WLS and Health and Retirement Study (MRS)to depict how the range of cognitive function in the WLS compares to a nationally representative distribution from the same cohort and other populations of interest. Then, we will use models of reciprocal causation to estimate how social and intellectual engagements at work and in leisure affect and are affected by cognitive functioning. We will study how cognitive variation among participants is implicated in their response to two health-related decision contexts of recent policy concern: their desired involvement in the process of making decisions about their medical treatment, and their behavior regarding the new and complicated Medicare prescription drug benefit. We will examine also how cognitive functioning is related to how much and how well respondents use and benefit from the Internet, especially in seeking information relevant for their health. Finally, we will explore and assess variations in cognitive functioning, social environments, and decision-making processes across the life course in relation to an assessment of APOE in the entire sample and in major subpopulations, such as those defined by gender, early cognitive ability, educational attainment, and economic and family roles.